Video signals are commonly defined in terms of a fixed set of primaries, such as those defined by the European Broadcast Union (EBU). Recently, wide-gamut displays have been developed that are capable of reproducing colors outside of the EBU-triangle, by either using more saturated primaries (e.g., using LED backlights) or by making use of more than three primaries in the backlight. See for example “High brightness direct LED-backlight for LCD-TV”, by West, R. S. et al., in: SID03 Digest, pp. 1262-1265, May 2003; “Wide color gamut and high brightness assured by the support of LED backlighting in WUXGA LCD-monitor”, by Sugiura, H. et al., in: SID04 Digest, pp. 1230-1233, May 2004; “Six-primary-color 23 inch WXGA LCD using six-color LEDs”, by Sugiura, H. et al., in: SID05 Digest, pp. 1124-1127, May 2005; and “Spectrum Sequential LC-TV”, by Jak, J. J. J. et al., in: Proc. IDW05, December 2005.
As a consequence, there exists a need for color processing algorithms with which the incoming video signal can be mapped to the reproduction gamut in such a manner that the potential of the wider color gamut of such new displays is better exploited. Similarly there exists a need for color processing algorithms with which the incoming video signal can be mapped to the reproduction gamut in the case where the input gamut comprises colors not included in the reproduction gamut. This is for example the case when rendering a video signal that is prepared for a wide-gamut display on a legacy CRT display. It is also the case when rendering video or images on small-gamut displays found in for example mobile phones and PDA's.
EP 0 723 364 A2 discloses an image enhancement method for optimizing intensity and saturation values. EP 1 443 456 A1 discloses a color image processing method including reducing saturation for a pixel at which the detected saturation is smaller than a predetermined threshold, and enhancing saturation for a pixel at which the detected saturation is greater than a predetermined threshold. However, both these methods relate to image enhancement within a same gamut and do not make use of the larger reproduction gamut available on new displays.
From color science, it is well known how signals represented on one set of primaries should be transformed to another set of primaries such that the underlying color information is preserved. Such a transformation is called a colorimetric mapping. Also, a simple and direct approach to making use of the wider color gamut is by applying no color processing at all (not even colorimetric mapping) and simply use the same drive values defined on the EBU-primaries for display.
EP 1 195 983 A2 discloses an image processing apparatus provided with a color-correction lookup table for chroma enhancement, which applies chroma-enhancement processing to a desired color point in the input image signal in an area where an output system gamut is wider than an input system gamut, and uses the color-correction lookup table for chroma enhancement according to the input image signal for performing the desired color correction. The color-correction lookup table for chroma enhancement does not perform the chroma-enhancement processing when the desired color point is close to achromatic color.